1. Field of the Invention
This invention relates to well completion fluids and more particularly to a new high-density fluid that may be employed without excessive corrosion or undue crystallization.
2. Description of the Prior Art
Drilling fluids have conventionally been used to maintain control during perforation, completion, or workover operations in oil and gas wells. Drilling fluids that have heretofore been employed for such purposes include mud, saltwater, water, or oil. The use of these fluids during drilling operations has per se been generally satisfactory and has actually enhanced drilling efficacy. However, the same materials have been employed during completion and workover operations with undesirable consequences.
For example, use of drilling muds during well perforations has frequently resulted in plugging of the perforations. Solids present in such drilling fluids have caused plugging and have made the completion process unduly complex, expensive and unreliable. Similarly, use of drilling muds and other drilling fluids as packer fluids has resulted in unwanted settling of solids. Moreover, drilling media may be somewhat corrosive under long-term, static operating conditions thereby further rendering them unsuitable for use other than as transitory drilling aids.
In an effort to overcome the foregoing problems, one approach that has been suggested is the utilization of high density salt solutions. One such solution is disclosed in U.S. Pat. No. 3,126,950, issued Mar. 31, 1964. This patent describes water solutions of zinc chloride and calcium chloride and indicates they have utility as well completion fluids at densities ranging between about 11-14 pounds per gallon ("ppg"). While the patent indicates that in theory zinc chloride/calcium chloride solutions having densities as high as about 17 ppg may be prepared, the patentees state that they "have found that solutions containing sufficient ZnCl.sub.2 to obtain densities above about 14 ppg cause significantly greater corrosion on the ferrous metals in the well." (Column 5, lines 29-32).
However, even with added corrosion inhibitors, these zinc chloride/calcium chloride solutions have had limited utility. Not only have severe downhole corrosion problems been encountered in their use, but significant corrosion of above ground equipment has also occurred as has injury to personnel. Moreover, the practical limitation of 14.0 ppg as in upper working limit for zinc chloride/calcium chloride solutions has made them unsuitable for deep drilling uses where higher densities are required to exert sufficient hydrostatic pressure to control the well.
Subsequently, it has been suggested that mixtures of calcium bromide and calcium chloride in water provide solids-free brines having densities as high as 15.1 ppg (Wendorff, "New Solids-Free High Density Brines Solve Many Workover and Completion Problems", Society of Petroleum Engineers Paper No. SPE 4788 (1974)). While the use of such calcium bromide/calcium chloride solutions has represented an improvement over the zinc chloride/calcium chloride solutions of U.S. Pat. No. 3,126,950, at densities approaching 15.1 ppg, the crystallization points of the calcium bromide/calcium chloride solutions are so high as to limit their utility to applications where low temperatures are not experienced. Moreover, the calcium bromide/calcium chloride mixtures cannot be safely or easily employed at densities in excess of 15.1 ppg (e.g., at densities as high as 18.0 ppg) required for higher pressured wells.
As a result, notwithstanding the demand for clear solutions as well completion and workover aids, the industry has continued to employ other approaches. Thus, emulsions and suspensions of materials such as iron carbonate have been employed. One popular fluid is a suspension of iron carbonate in a highly viscosified fluid. Such a system purports to be acid soluble such that, after use, acidification of the system results in dissolution of residual suspended iron carbonate particles. However, this approach has been unsatisfactory because up to about 8% of the iron carbonate is insoluble in acid, and difficulties have also been encountered in obtaining acid penetration in the difficult to get to areas in which the iron carbonate particles collect.
In short, the art has long sought a clear, high-density (i.e., up to about 18.0 ppg) corrosion controlled fluid suitable for use as a well completion, packing, and perforation medium, and the primary object of this invention is to provide such a fluid.
Another object is to provide a series of solutions which exhibit crystallization points which may be tailored to prevailing weather conditions on an economically advantageous basis.
A still further object is to provide a fluid of the character described having a low, controlled, corrosion rate.
A still further object is to provide a fluid of the character described which is not only non-corrosive when used in downhole applications but which is also free from above the ground corrosion problems and which may be safely used by personnel.
A still further object is to provide a fluid of the character described which is usable on a long term basis in oil and gas wells.